Magnetic amplifiers



Jan. 17, 1956 J. A. CRAWFORD MAGNETIC AMPLIFIERS Filed Aug. 12, 1952 INVENTOR. JACK A. CRAWFORD ATTORNEYS United States Patent 2,731,521 MAGNETIC AMPLIFIERS Jack A. Crawford, China Lake, Calif., assignor t0 the United States of America as represented by the Secretary of the Navy Application August 12, 1952, Serial No. 304,049

2 Claims. (Cl. 179-171) (Granted under Title 35, U. S. Code (1952), see. 266) The present invention relates to improvements in magnetic amplifiers involving shunting the rectifiers in a manner to make the amplifier independent of the rectifier characteristics. In particular, by heavily shunting the rectifiers in accordance with this invention, their back resistance becomes unimportant and the forward and back resistances of the rectifiers can be accurately compensated for by cross-over turns. The result of shunting the rectifiers is that their forward and back resistances are in effect constant.

Magnetic amplifiers are coming into wider usage for the reason that they are more rugged and less subject to failure than vacuum tube amplifiers and, therefore, preferable in many applications where these characteristics are desired.

Magnetic amplifiers, as commonly known, ordinarily involve a D. C. primary winding through which a signal voltage is passed and which is associated with a pair of magnetic cores and A. C. secondary windings. The D. C. winding saturates the magnetic cores and thereby controls the current in the secondary windings, the device acting as an amplifier. The secondary windings have rectifiers in circuit therewith arranged so that the current flow in them assists the primary winding in magnetizing the cores. Rectifiers, as commonly used, such as barrier layer rectifiers, do not have constant forward and back resistances, these quantities being highly nonlinear functions of voltage, temperature, and sometimes duty cycle. In addition, the rectifiers commonly vary quite widely in characteristics from unit to unit so that the use of cross over turns to compensate for back current through the rectifiers is ineffective. That is, the variability of the characteristics of the rectifiers, as mentioned above, makes it impossible to always have the proper number of cross-over turns for all conditions. Cross-over turns are additional windings associated with the magnetic cores to assist in their magnetization and to offset the demagnetization of the back current through the rectifiers.

Both negative and positive feedback have heretofore been used in magnetic amplifiers. The present invention utilizes negative feedback with heavily shunted rectifiers so that their back resistance is unimportant whereby the operation of the magnetic amplifier is substantially independent of rectifier characteristics.

The primary object of the invention is to provide an improved magnetic amplifier in which the operation of the amplifier is independent of rectifier characteristics.

Another object of the invention is to provide a mag netic amplifier having cross-over windings and having heavily shunted rectifiers whereby the variable characteristics of the rectifiers are compensated for.

Another object of the invention is to provide a magnetic amplifier having a cross-over winding in series with each secondary winding, a rectifier in series with each set of windings and a resistor connected in shunt across the two circuits whereby to form a shunt for each of the rectifiers.

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Further objects and numerous advantages of the invention will become apparent from the following detailed description and annexed drawings wherein:

Fig. 1 is a circuit diagram of a self feedback magnetic amplifier as known in the prior art.

Fig. 2 is a circuit diagram of a self feedback amplifier with cross-over windings.

Fig. 3 is a circuit diagram of a magnetic amplifier similar to that of Fig. 2 with each of the rectifiers resistively shunted.

Fig. 4 is a circuit diagram of a magnetic amplifier similar to that of Fig. 3 but with the rectifiers shunted by a single resistor connected across the circuit of the two sets of windings.

Referring to Fig. 1 of the drawings, numeral 10 designates a primary D. C. winding. The magnetic amplifier includes two magnetic cores as indicated at 11 and 12. Associated with the primary windings and magnetic cores are secondary windings 13 and 14. Numeral 17 designates an A. C. voltage source connected to the secondary windings 13 and 14. The connection to the winding 13 is through rectifier 18 and the circuit for energizing winding 14 with similar polarity during the other half of the cycle includes rectifier 19 and the wire 20. Wire 21 connects the midpoint of windings 13 and 14 to the voltage source 17 through the load resistor 22. In this 'circuit the secondary windings 13 and 14 are similarly energized during alternate half cycles of the voltage source 17. However, the gain available is greatly lowered due to the rectifier back resistance which permits the flow of available back currents which tend to demagnetize the cores of the device thereby reducing the gain. The addition of a more positive feedback is now desirable to increase the gain. The additional feedback can be secured by cross-over windings as shown at 25 and 26 in Fig. 2 which are in series respectively with the secondary windings 13 and 14. The cross-over windings assist in magnetizing the magnetic cores of the device, but, as pointed out in the foregoing, they can only compensate for the rectifier forward and back resistances if these resistances are constant. These resistances are, however, not constant, as pointed out in the foregoing, they being highly non-linear functions. To compensate for this variability, the shunt resistors 27 and 28 are placed in the circuit as shown in Fig. 3 shunting rectifiers 18 and 19. In addition to the shunts, the crossover windings in this figure include a greatly increased number of turns. The use of the shunts makes the forward and back resistance of the rectifiers unimportant and the effect is the same as if it were constant. The resistance of the shunt is constant and can be accurately compensated for by adjusting the number of turns and cross-over windings.

On one-half cycle of the voltage source 17, the circuit is through the wire 20, rectifier 18, winding 13, winding 25 and load resistor 22 back to source 17. On the other half-cycle the circuit is from the source 17 through the load resistor 22, wire 21, winding 26, Wind ing 14, rectifier 19 and wire 23 back to the source 17.

In the circuit of Fig. 4 only one shunt resistor is utilized as shown at 30 which connects the circuits of the two sets of windings as shown adjacent the rectifiers. The resistor 30 forms a shunt circuit for each of the rectifiers. In this circuit arrangement, the shunt is in such a position as to make the division of current through the two sets of windings independent of rectifier back resistance. That is any back current through a rectifier will be negligible as compared to that through the shunt. As can be observed in this circuit, the crossover windings can be adjusted to accurately compensate for rectifier resistance with resultant increase in available amplifier gain and with the result that the operation of the circuit is independent of rectifier characteristics. The advantage of the circuit is that it makes possible the use of cheaper rectifiers. Another property of the shunted rectifier circuit is the low back voltages impressed upon the rectifiers. The shunting resistors limited the back voltage that can be developed across the rectifiers to 20 percent of the supply voltage or less. This low back voltage allows the use of germanium diode rectifiers under conditions of voltage and temperature which would be impossible with previous circuits.

The foregoing disclosure is representative of a preferred embodiment of my invention and it is to be understood that various modifications, adaptations and alternatives may be adopted by those skilled in the art. The scope of the invention is to be determined in accordance with the claims appended hereto.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. In a magnetic amplifier, a pair of magnetic cores, a primary winding on said cores and having a pair of terminals for connection to a D. C. source, a secondary circuit having a pair of parallel branch circuits and a pair of terminals for connection to an A. C. source, each of said branch circuits comprising a secondary winding, a cross-over winding and a rectifier connected in series, each of said cores having thereon the secondary winding in one branch circuit and the cross-over winding in the other branch circuit, the rectifiers being arranged so that the branch circuits are energized on alternate half cycles, resistance means shunting said rectifiers and having electrical resistance substantially less than the back resistance of said rectifiers whereby known back currents will flow through said branch circuits, each of the crossover windings being of such size and so Wound on the core relative to the secondary winding on the same core that the flux produced by the current flowing through a given cross-over winding during said alternate half cycles will compensate for the flux produced by said known back current through the secondary winding on the same core.

2. The arrangement according to claim 1 wherein said resistance means comprises a resistor connected in shunt with each rectifier.

Retarences Cited in the file of this patent UNITED STATES PATENTS 2,126,790 Logan Aug. 16, 1938 2,563,740 Parker Aug. 7, 1951 FOREIGN PATENTS 125,633 Australia Oct. 23, 1947 OTHER REFERENCES A. I. E. E. Miscellaneous Paper on Magnetic Amplifiers No. 50-93 by Geyger, December 1949, 179-171- MA. (Copy in Div. 69.) 

